Topic 2.7 Notes – VSEPR and Hybridization

1. What VSEPR and Hybridization Explain

Covalent bonds form when atomic orbitals overlap and electrons are shared. Those electron pairs repel each other because of Coulombic repulsion. VSEPR theory says electron domains arrange themselves to minimize that repulsion around a central atom.

To get the full picture, you combine:

• Lewis structure
  • Shows connectivity
  • Shows lone pairs
  • Shows multiple bonds (bond order)
• VSEPR
  • Predicts electron geometry (all electron domains)
  • Predicts molecular geometry (positions of atoms only)
  • Predicts approximate bond angles
• Hybridization
  • Describes the arrangement of orbitals on the central atom
  • Connects directly to ideal bond angles
  • Limited to sp, sp², sp³ on the AP exam

From this chain, you should be able to determine:

• Molecular geometry
• Bond angles
• Bond order → relative bond length and bond energy
• Dipole moment (polar or nonpolar)
• Hybridization

Everything starts with a correct Lewis structure. If that’s wrong, everything downstream is wrong.

2. All Molecular Geometries You Need to Know

When you’re given a molecule, the logic always flows the same way:

1. Draw the Lewis structure.
2. Count electron domains around the central atom (bonding regions + lone pairs).
   • A double or triple bond counts as one domain.
3. Identify electron-domain geometry.
4. Ignore lone pairs to name the molecular shape.

A. 2 Electron Domains

• Electron geometry: Linear
• Molecular shape: Linear (MX₂)
• Bond angle: 180°
• Hybridization: sp

Example: $\ce{CO2}$

B. 3 Electron Domains

• Electron geometry: Trigonal planar

Lone pairs repel more strongly than bonding pairs, so they compress bond angles.

C. 4 Electron Domains

• Electron geometry: Tetrahedral

This is the family students mix up most. Remember: electron geometry stays tetrahedral even when the molecular shape changes.

D. 5 Electron Domains

• Electron geometry: Trigonal bipyramidal
• Angles: 90°, 120°, 180°

Lone pairs prefer equatorial positions because that creates fewer 90° repulsions. In the trigonal bipyramidal row of the chart below, notice the three equatorial positions in one plane and the two axial positions above and below.

VSEPR molecular geometry summary chart

You only need the shapes. Hybridization with d orbitals is not tested.

E. 6 Electron Domains

• Electron geometry: Octahedral
• Angles: 90°

When there are two lone pairs, they sit opposite each other to minimize repulsion.

3. Sigma and Pi Bonds and Bond Order

Bond formation happens by orbital overlap.

Sigma (σ) Bonds

• Head-on overlap
• Stronger overlap → higher bond energy
• Every single bond is 1 σ
• Allows rotation

Pi (π) Bonds

• Side-by-side p orbital overlap
• Weaker than σ
• Prevents rotation

Bond composition:

• Single = 1σ
• Double = 1σ + 1π
• Triple = 1σ + 2π

The diagram below shows head-on overlap forming σ bonds on the left and side-by-side p orbital overlap forming a π bond on the right.

Sigma vs. pi orbital overlap

Important trends:

• Higher bond order → shorter bond length
• Higher bond order → greater bond energy
• π bonds create rigidity → geometric (cis/trans) isomers

On tests, they love asking which C-C bond is shortest. Triple wins every time.

4. Hybridization and Bond Angles

Hybridization matches the number of electron domains:

Count domains. That number gives you hybridization.

Lone pairs change molecular shape, but hybridization depends on total domains.

5. Bond Length, Bond Energy, and Dipole Moment

Bond Length

Depends on:

1. Bond order
   • Triple < Double < Single
2. Atomic radius
   • Larger atoms → longer bonds
   • Down a group → bonds get longer

If bond order is the same, compare atomic size.

Dipole Moment

A molecule is polar if bond dipoles do not cancel.

Think in three steps:

1. Identify polar bonds (electronegativity difference).
2. Determine molecular geometry.
3. Add dipoles as vectors.

The diagram below compares a linear molecule, CO₂, with a bent molecule, H₂O.

Dipole cancellation in CO₂ vs. net dipole in H₂O

CO₂ is linear, so the two C=O bond dipoles are equal and opposite and cancel. H₂O is bent, so the O-H bond dipoles add to produce a net dipole.

Symmetry cancels dipoles. Lone pairs often create asymmetry.

Linear with identical outer atoms is nonpolar. Bent and trigonal pyramidal are usually polar.